Microcellular foams for use in packaging, containers, films, and the like have been reported. See U.S. Pat. No. 4,473,665-Martini-Vredensky, Suh and Waldman; U.S. Pat. No. 4,761,256-Hardenbrook, et al; and Ph.D. Thesis, MIT 1986, Jonathan Colton. In this prior art, small cell size is obtained by quenching the material during the formation of the cells at the appropriate time to control the size of the cells.
Various methods of density reduction have been investigated. These approaches cause a nearly linear property reduction with bulk density. These techniques typically use a chemical blowing agent (CBA) to create foam structure in polyethylene. Most used two or three layer coextrusions with foam comprising the "core". The best material made using this approach still has significant property degradation with density reduction. However, the data shows that as cell size decreases and cell uniformity increases film properties suffer far less. This suggests that very fine, well controlled cells could reduce bulk density while retaining acceptable properties.
Recently several articles have disclosed research into very fine cell foams or microfoams, with cells between 3 and 10 microns. This shows the interesting result that microcells improve rather than decrease impact properties of plaque molded samples. These small foam cells prevent crack growth in the matrix by dissipating the force at the crack tip over a much wider area similar to the effect of rubber particles in high impact polystyrene (HIPS). Typical cell sizes for "fine cell" styrene foams are 100+ microns and, although they have better impact properties than coarser foam structures, they are poorer than the styrene matrix. Microfoaming creates an impact resistant structure that has better properties than the unreduced matrix.
The method reported in the literature to create a microcellular foam is to add a material to the molten polymer at its solubility limit at the polymer melt temperature. This material is molded into a plaque sample and rapidly quenched. Plaques are put into a Paar bomb and saturated with a gas (nitrogen, carbon dioxide) at elevated pressures for times sufficient to ensure gas diffusion into the polymer matrix. Samples are then quickly heated to reduce matrix viscosity, increase gas volatility in the matrix and create micro foam. A material added at its solubility limit will supposedly create nucleating sites for foam cells as the molten polymer cools and the material precipitates from solution. Since the material is soluble and hence uniformly distributed, the nucleation sites are uniformly distributed throughout the matrix. The site size is very small since the nucleation occurs on the forming precipitate which can be clumps as small as several molecules. This nucleation mechanism has been referred to as "pseudo-homogeneous" since the effect of the added material as a typical nucleant is unclear. Zinc stearate and stearic acid have been used as nucleants in this manner.
Much of the work recently reported in the literature originated at MIT and Dr. Nam Suh. Several theses were published (The Production and Analysis of Microcellular Foam, J. E. Martini, MS thesis, 1981; The Processing of Microcellular Foam, F. A. Waldman, MS thesis, 1980; The Nucleation of Microcellular Thermoplastic Foam, PhD thesis, 1985) and later reported in various journal articles. The aforementioned U.S. Pat. No. 4,473,665 titled "Microcellular Closed Cell Foams and Their Method of Manufacture; Saturation With Inert Gas, Depressurization and Quick-Cooling" is a process patent describing the microfoam method. Microfoam work at MIT spans about ten years and includes work in both crystalline as well as amorphous polymers. The bulk of the work reported has been in various styrene matrices due to processing ease and subsequent characterization.
Polycarbonate foams, polyetherimide (Ultem.RTM.) foams, polypropylene oxide/polystyrene or HIPS foams, Noryl.RTM., are also in use. See, for example, U.S. Pat. No. 4,598,101.